Implantable medical devices are commonly used to treat patients suffering from a variety of medical conditions. Implantable medical devices can be used to treat any number of conditions such as pain, incontinence, movement disorders such as epilepsy and Parkinson's disease, and sleep apnea. Additional therapies appear promising to treat a variety of physiological, psychological, and emotional conditions.
Such implantable medical devices include implantable drug infusion pumps and implantable electrical stimulators. Implantable drug infusion generally infuse a specific amount of a drug contained in a reservoir of the implantable drug infusion pump to the patient in accordance with a scheduled program assigned by a medical professional. Implantable electrical stimulators generally deliver an electrical stimulation to a nerve or other tissue of a patient, e.g., to control pain. In both cases, electrical power may be required to operatively drive the therapeutic componentry associated with each device, e.g., a stepper motor for a drug infusion pump and a pulse generator for an electrical stimulator. In addition, both types of implantable medical devices may also require electrical power for ancillary control and communication circuitry.
An implantable battery is often used as a source of electrical power for an implantable medical device. It is also common to use a rechargeable battery in such an implantable medical device in order to achieve maximum life and small size of the implantable medical device as well as provide robust functions requiring an ample electrical power source. Such implantable medical devices depend upon periodic charging or recharging of the rechargeable battery or batteries in order to maintain continued function of the implantable medical device.
Various techniques and procedures have been established to perform and achieve charging or recharging of an implanted battery of an implantable medical device typically through the use of electromagnetic coupling.
A battery of a typical implantable medical device has a voltage when fully charged. As the battery of the implantable medical is depleted through use the voltage of the battery will typically decline. At a suitable point in time, the battery may then be charged or recharged, e.g., through such electromagnetic coupling, to restore the capacity of the battery.
It is common to utilize the voltage of battery of an implantable medical device as a barometer as to when to charge or to recharge the battery. Typically, the voltage of the battery is monitored and when the voltage of the battery reaches a predetermined voltage, the recharge voltage, the battery is then suitably recharged. The implantable medical device, either alone or in conjunction with an external programmer or control device or monitor, typically signals the user, e.g., the patient, when or following the point in time when the recharge is reached in order that the patient may take appropriate steps to have the battery recharged.
This recharge voltage is typically selected in order to provide the patient with a reasonable useful life, i.e., reasonable functional time between charges, as well as to provide a safety margin or time between the time at which the recharge voltage is reached and a time when the voltage of the battery declines to a point at which the implantable medical device is no longer functional. The time period between the point in time when the recharge voltage is reached and the point in time when the implantable device may no longer function is typically referred to as the compliance period, i.e., the period of time in which the patient should comply with recharging of the battery without risking operation of the device from a low voltage from the battery. The compliance period generally should be long enough to allow the patient a reasonable period of time to accomplish recharging. For example, if special equipment is needed for recharging, then the compliance period should be reasonably long enough to allow the patient in normal situations to obtain and utilize the special equipment needed for recharging.
Generally, the voltage of a fully charged battery when new can be determined within reasonable accuracy. Also, the recharge voltage can be selected for a new battery to achieve a reasonable operational period, i.e., the time period between charges, and also to allow for a reasonable compliance period. It is recognized that a lower recharge voltage will generally provide a longer operational period but a shorter compliance period. The converse is also true. A higher recharge voltage will generally provide a shorter operational period but a longer compliance. Thus, the recharge voltage is generally set at a compromise level to ensure a reasonable operational period and a reasonable compliance period.
However, as the battery of an implanted medical device ages the fully charged voltage will typically decline and, more importantly, the capacity of the battery between its “fully charged” state and its “needs recharging” state may be drastically reduced. For example, a new battery may have a 300 ampere-hour capacity while an aged battery may only have a 30 ampere-hour capacity. Thus an aged but “fully charged” battery may have a lower fully charged capacity and voltage than a “fully charged” battery that is brand new. Thus, the capacity and voltage between the “fully charged” state and the time at which the battery reaches its recharge voltage may be significantly less for an aged battery. As a result, the operational period of an aged battery may be significantly less than the operational period of a brand new battery.
Also, as the battery of an implanted medical device ages the time between the time the battery recharge voltage is reached and the time the lowest operational voltage is reached may also decrease. As a result, the compliance period of an aged battery may be significantly less than the compliance period of a brand new battery.